The functionality afforded mobile radio communication devices such as cell phone handsets has been increasingly defined by reference to the Third Generation Partnership Protocol (3GPP) proposals and standards and which has focused most recently upon ongoing improvements to the Universal Mobile Telecommunications System (UMTS) such as the Long Term Evolution (LTE) ongoing improvement thereto.
In order to access services in 3GPP networks, mobile radio communications device User Equipment (UE) need to be registered to the network. Once the UE has been registered with a network, session information for the UE of an active session is maintained in a context for the UE.
In some circumstances, the session information of the active session maintained in the context may be lost or become unrecoverable. This can occur, for example, due to inter-node mobility, or because the network has implicitly detached the UE due to there being no communication from the UE to the network for a period of time. In these instances, when the UE attempts to access the network, using, for example, a registration request such as a tracking area update (TAU) request in LTE systems, or a routing area update (RAU) request in UMTS systems, or a service request in LTE or UMTS systems, the network will reject the request from the UE indicating a specific cause for the rejection to the UE. Upon reception of a signal from the network indicating the specific cause for rejection, the UE may attempt to re-establish connection with the network (or “re-attach” to the network).
Examples of causes for rejection of a request in LTE systems are as follows:
Cause #9—UE identity cannot be derived by the network.                This Evolved Packet System Mobility Management (EMM) cause is sent to the UE when the network cannot derive the identity of the UE from the Globally Unique Temporary Identity (GUTI)—S-Temporary Mobile Subscriber Identity (S-TMSI)—P-TMSI and RAI (Routing Area Identity), e.g. no matching identity/context in the network or failure to validate the identity of the UE due to integrity check failure of the received message.        
Cause §10—Implicitly detached.                This EMM cause is sent to the UE either if the network has implicitly detached the UE, e.g. after the implicit detached time has expired, or if the EMM context data related to the subscription does not exist in the Mobility Management Entities (MME)T e.g. because of a MME restart.        
In UE which can support LTE and Universal Mobile Telecommunications System (UMTS), there may be instances where the UE is initially operating under UMTS but then transfers to LTE. In such instances, when the UE transfers from UMTS to LTE with no Packet Data Protocol (PDP) context established, the network will reject a tracking area update request from the UE in order to initiate an attempt by the UE to re-establish connection with the network (“new attach”). This new attach triggers the establishment of a default bearer which is necessary in LTE systems.
In this instance, an example of a cause for rejection of a request is as follows:
Cause #40—No EPS bearer context activated.                This EMM cause is sent to the UE, if during a tracking area updating procedure the MME detects that there is no active EPS bearer context in the network.        
As noted above, when the UE receives a rejection to its request to access the network, the reason for the rejection (i.e. “cause”) is indicated to the UE. The type of causes of rejection will depend upon the request sent by the UE to the network (e.g. causes #9 or #10 or #40 during TAU procedures in LTE systems, or causes #9 or #10 during service request procedures in LTE systems, or causes #9 or #10 during RAU procedures in GSM/UMTS systems, or causes #9 or #10 during service request procedures in UMTS systems). Upon receipt of the rejection and indication of the cause, the UE is operative to attempt a re-attach with the network.
The re-attach process may be undertaken using a non-access stratum (NAS) signalling connection. Some current network systems require UE to implement the re-attach process using an existing NAS signalling connection. However, other current network systems require UE to implement the re-attach process using a new NAS signalling connection once the existing NAS signalling connection has been released by the network. Also, some current UE re-use an existing NAS signalling connection as part of the re-attach process, whilst some other current UE await release of the existing NAS signalling connection before implementing the re-attach process using a new NAS signalling connection.
There may be delays in re-establishing connection between UE and the network in situations where network systems and UE implement one of the re-attach processes described above. Particular examples of situations in which delays can occur are illustrated in FIGS. 1 and 2, which are discussed briefly below, but are described in more detail later.
FIG. 1 illustrates a schematic process flow for a first situation in which a delay can occur in a re-attach process. In this first situation, a UE is operative to implement a re-attach process using an existing NAS signalling connection. Thus, after receiving from a network device a rejection in response to an attempt by the UE to access the network, the UE sends a request to the network device for re-establishment of connection. However, at around the same time, the network releases the NAS signalling connection. Thus, the request for re-establishment of connection, which is sent by the UE during the re-attach process, is not received by the network device because the NAS signalling connection was released by the network device before the request reached the network device. The UE, having not received any acknowledgement from the network device, will determine that the NAS signalling connection has been released by the network device. Upon such determination, the UE is operative to initiate a timer. The UE postpones the re-attach until a predetermined amount of time (e.g. 15 seconds), as measured by the timer, has elapsed. When the predetermined amount of time has elapsed, the UE initiates establishment of a new NAS signalling connection with the network device, and then is operative to attempt the re-attach process using the new NAS signalling connection. The time between an initial attempt at re-establishment of connection and a subsequent attempt introduces a delay into the re-attach process.
FIG. 2 illustrates a schematic process flow for a second situation in which a delay can occur in a re-attach process. In this second situation, a UE is operative to implement a re-attach process using a new NAS signalling connection. Thus, after receiving from a network device a rejection in response to an attempt by the UE to access the network, the UE waits for the network device to release the existing NAS signalling connection (because it is not operative to use the existing NAS signalling connection). However, the network device may not be configured to release the NAS signalling connection because it is operative to receive a request for re-establishment of connection via the same existing NAS signalling connection. The existing NAS signalling connection may eventually be released locally by the UE. For example, a guard timer in the UE may be started when the rejection from the network device is received by the UE. When the guard timer denotes that a predetermined amount of time has elapsed, the UE is operative to locally release the existing NAS signalling connection. Once the existing NAS signalling connection has been released, the UE is operative to establish a new NAS signalling connection with the network device. The UE can then attempt the re-attach process using the new NAS signalling connection. Again, the time between an initial attempt at re-establishment of connection and a subsequent attempt introduces a delay into the re-attach process.
The above-described examples illustrate the different implementations which exist with regards to the usage of an existing NAS signalling connection or otherwise during a re-attach process.